Derivation of surface direct beam spectral albedo for a wide range of incident angles from spectral reflectance measurements under overcast conditions

Measurements made under overcast conditions can be used to derive surface direct beam spectral albedo for a wide range of solar incident angles. The derivation is based on the reciprocity between directional-hemispherical and hemispherical-directional reflectance values. Under overcast conditions, the sky has a nearly constant intensity pattern. This approximately fits the condition for the reciprocity. Therefore, the directional spectral reflectance measured at a certain viewing angle can be converted to the direct beam spectral albedo at the incident angle that equals the given viewing angle. During our two summer sea ice cruises in the Southern Ocean, we have measured surface directional spectral reflectance at 57 ice stations. Among those stations, there are quite a few stations where we had overcast conditions. The conversion can significantly expand the database of the snow and sea ice surface direct beam spectral albedo in the Southern Ocean and helps improve sea ice surface albedo parameterization in the global circulation models